Several technologies are available to notify users of events or new information. Typically, these technologies are based on email lists, bulletin boards, publish-subscribe systems, database triggers and filtering systems. A core issue in all of these systems is how the message to be sent is matched with interested subscribers. Email distribution lists and bulletin boards, for example, provide a fixed taxonomy of topics of interest. People subscribe to receive notifications for one or more topics by joining the email list or the bulletin board on that topic.
Publish-subscribe systems are also often organized with fixed taxonomies of topics. These systems are sometimes called topic-based (or type-based) publish-subscribe systems in the literature. Another category of publish-subscribe system is called the content-based publish-subscribe system. In these systems, attribute-value pairs in the message published are matched against user-specified constraints that are conjunctions of relational expressions over some of the attribute-value pairs (for example, item=car and price <$10,000) or, more generally selection queries in a normal form such as conjunctive normal form (for example, ((item=car) or (item=motorcycle)) and (price <$10,000)).
While such publish-subscribe systems provide an effective mechanism for notifying users of events or new information, they suffer from a number of limitations which, if overcome could make such publish-subscribe systems better suited for enterprise applications. For example, currently available publish-subscribe systems only allow users to enter subscriptions based on static attributes. A need therefore exists for improved publish-subscribe systems that allow users to enter subscriptions based on one or more dynamic attributes.